The invention relates to digital watermarking, and more particularly relates to watermark detection in multimedia content (e.g., still image, video and audio signals).
The reader is presumed to be familiar with digital watermarking technology. See, e.g., co-pending application Ser. No. 09/503,881, filed Feb. 14, 2000, entitled Watermark Reader and Embedder, the disclosure of which is incorporated by reference.
One objective of watermark detectors is to reject unmarked signals (e.g., image, audio, video signals) at the earliest possible stage of detection. The detector may conclude that a signal is unmarked based on quantitative evidence of the watermark (or lack thereof) in a signal suspected of having a watermark. The signal might be an unmarked component of a marked signal, or simply an unmarked signal. Also, in some cases, the signal, though previously marked, may appear to be unmarked due to removal or degradation of the watermark. By accurately identifying an unmarked signal at an early stage, the detector can avoid unnecessary processing. Also, the apparent absence of a watermark may trigger some action (or prevent an action) such as providing output indicating that the signal has been tampered with or controlling processing of the signal (e.g., preventing copying, playing or recording in copy protection applications).
A related objective of a watermark detector is measuring the strength of a watermark signal. Based on the watermark strength, the detector can assess whether a suspect signal has a valid watermark, and the extent to which a signal has been transformed. The detector can also determine the likelihood that a suspect signal includes a valid watermark or recoverable watermark message. Such an evaluation helps the detector allocate its processing resources on portions of the suspect signal that are likely to contain a valid watermark or recoverable watermark message.
The cited application describes a variety of techniques for detecting a watermark. Some of these techniques correlate attributes of a watermark signal with a signal suspected of containing a watermark. By measuring the extent of correlation, a watermark detector assesses whether a watermark is present, and in some cases, determines its orientation in the suspect signal. Related techniques detect a watermark signal by at least partially decoding a message from the suspect signal and then comparing attributes of the message with expected attributes to assess the likelihood that a watermark signal is present. These and other techniques may be used to compute a detection value that quantifies the likelihood that the suspect signal has a watermark.
One aspect of the invention is a method of using detection values ascertained from signals suspected of being watermarked to control the detection process. The detection values may be used to reject unmarked signals. In addition, they may be used to refine the detection process by focusing the detector on signals or portions of signals that are likely to contain a watermark and/or a recoverable watermark message. Each portion of a suspect signal may be defined by an orientation parameter (or set of parameters like rotation, scale, origin, shear, differential scale, etc.). Also, each portion may represent different orientations of the suspect signal, or a component of the signal.
Another aspect of the invention is a method for using two or more detection metrics to control the detection process. The multiple metrics could be derived from independent measurements in multiple stages or could be different features of the same measurement. Each detection metric evaluates detection values to control detection actions. One type of detection metric is a screen used to evaluate suspect signals or portions of a suspect signal for the presence of a watermark. Each stage evaluates detection values to assess whether a suspect signal, or portion of it, is marked.
Another aspect of the invention is a method for using absolute and relative detection measures to assess whether a suspect signal is marked. An absolute measure of detection represents quantitative evidence of a watermark signal in a suspect signal, and is usually evaluated independently from other detection values. A relative measure is based on the relative values of two or more detection values, which may be relative or absolute measures. A relative measure may be implemented by computing absolute detection values for different portions of a suspect signal and then computing a relative detection value as a function of the absolute detection values.
Both absolute and relative detection values may be evaluated relative to desired limits or thresholds to determine an appropriate action. One action is to reject the candidate signal associated with the detection value as being unmarked. Another action is to use the detection values to direct further actions of the detector. One advantage of using both absolute and relative detection values is that they usually contain complementary information. This complementary information helps in improving the watermark screening and detection process.
In one implementation, a detector computes detection values for different orientation parameter candidates, sorts the detection values in terms of likelihood of representing a valid watermark, and then takes a ratio of a top detection value relative to one or more lesser detection values. The orientation parameter candidates define an approximate orientation and/or location of a watermark in a suspect signal, and as such are associated with a portion of a suspect signal.
The foregoing and additional features and advantages of the present invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.